Embodiments of the present invention relate to an RF probe for coupling a probe signal out from a transmission line of a circuit under test. Further embodiments relate to an automated test equipment comprising an RF probe and a receiver. Further embodiments relate to a method for coupling a probe signal out from a transmission line of a circuit under test. Some embodiments relate to a non-invasive RF in-circuit probe having extended operating frequency.
FIG. 1a shows a block diagram of a conventional non-invasive RF probe 10. The RF probe comprises a probe pin/contact block 12 and a high impedance element/circuit 14 connected in series between the probe pin/contact 12 and an external receiver 16. The high impedance element/circuit 14 can be a high-valued series resistor or a field effect transistor.
FIG. 1b shows an illustrative view of a tip of a conventional non-invasive RF probe 10 comprising a ground pin 12_1 and a signal pin 12_2.
The high input impedance characteristic of typical high impedance RF probes for in-circuit testing is determined at low frequencies mainly by the input impedance of the frontend circuitry of the probe. When the probe's input impedance is high, the circuit under test is unperturbed and therefore the probe is considered non-invasive.
While a typical probe retains its non-invasive characteristic at low frequencies, the minimum manufacturable length L, measured from the probe tip (tip of the probe pins 12) to the high impedance element/circuit 14 of the RF probe 10, becomes a limiting factor when using the probes at high frequencies. When this length L becomes comparable to the wavelength of the operating frequency, a behavior called impedance transformation converts a previously high input impedance of the RF probe 10 into a low impedance. This causes it to disturb or be invasive to the probed device under test (DUT) line, as will become clear from the probed DUT line frequency response shown in FIG. 2. FIG. 2 shows in a diagram the probed DUT line frequency response for a conventional non-invasive RF probe 10 having a length L of 4.5 mm. In FIG. 2, the ordinate denotes the insertion gain (negative insertion loss) in dB, where the abscissa denotes the frequency in GHz. Thereby, the RF probe 10 is considered non-invasive (having a high input impedance) in the frequency region where the insertion loss is smaller than 1.5 dB, wherein the RF probe is considered invasive in the frequency region where the insertion loss is greater than 1.5 dB.
The highest operating frequency of the probe 10 is therefore constrained by the practical limits of the available micromachining technologies, in making the probe pin 12 as short as possible, and by electronics assembly techniques, in placing the probe circuitry 14 as close as possible to the probe tip.
A commercial high impedance active probe is the 85024 from Agilent Technologies. A short fixed-pin probe leads to an active circuitry that has a high input impedance. The prescribed frequency of operation is limited to 3 (GHz.
The RealProbe107 from Vectra has a higher operating frequency compared to the probe 85024 from Agilent Technologies. The probe comprises a coupling loss of 20 dB and therefore is presumably a passive probe. The probe can work until 7 GHz with an insertion loss of 1.5 dB on the probed DUT line.
The RealProbe 109 from Vectra has a similar architecture as the RealProbe107, except that the probe pin is constructed on the PCB thus making the length L shorter. The probe is advertised to work until 18 GHz. The use of the PCB as a probe pin could compromise its long term reliability when the probe is used continuously.
The RF probe 1205 from Aeroflex Corporation is similar in architecture as the probe from Agilent Technologies but with several grounding suggestions. It is limited in operation until 4 GHz.
Furthermore, U.S. Pat. No. 4,853,627 shows a wafer probe constructed with a probe pin and a PCB, placed on a support membrane, containing a high impedance amplifier component.
Moreover, U.S. Pat. No. 5,821,758 shows an RF in-circuit method for non-invasive measurements using signal redirection. Thereby, the approach of using 2 fingers in measurement necessitates a relatively large amount of printed circuit board space on the DUT, and the use of removable passives in creating contacts to redirect the signal can potentially lead to inconsistent measurements.